How to extract weights from SimpleWeightedEdge (from kruskal_mst)

Specific Domains Graphs
,
1

I’m trying to figure out how to interact with a minimal spanning tree object returned by Graphs.kruskal_mst(). I just can’t figure out how to extract the resulting edge wights (or the edges).

Here’s a minimal code to show the issue:

using Distributions
using Graphs
using SimpleWeightedGraphs
using Distances

M = reshape(rand(Normal(), 100), 10, 10)
g = SimpleWeightedGraph(pairwise(Euclidean(), M, dims = 1))
my_mst = Graphs.kruskal_mst(g)

# when inspecting the object
julia> my_mst
9-element Vector{SimpleWeightedEdge{Int64, Float64}}:
 Edge 2 => 8 with weight 2.3668794009593306
 Edge 1 => 10 with weight 2.4514610105905223
 Edge 1 => 9 with weight 2.4717071330626306
 Edge 6 => 8 with weight 2.714304985226182
 ⋮
 Edge 3 => 9 with weight 3.507507419474672
 Edge 4 => 9 with weight 4.113153232605692
 Edge 1 => 7 with weight 4.1630221327329044

julia> my_mst[1]
Edge 2 => 8 with weight 2.3668794009593306

My goal here is just to get a vector of the edge weights. I’ve tried a bunch of thing, treating it as a dictionary, trying to convert it back to a graph, trying to extract the edges, but nothing works. I’m sure it’s a simple solution but I can’t track it down.

2

Hi there!
Don’t hesitate to ask the REPL for help, it is my best friend in such cases.

julia> typeof(my_mst[1])
SimpleWeightedEdge{Int64, Float64}

help?> SimpleWeightedEdge
search: SimpleWeightedEdge SimpleWeightedGraphEdge SimpleWeightedDiGraphEdge

  No documentation found.

  Summary
  ≡≡≡≡≡≡≡≡≡

  struct SimpleWeightedEdge{T<:Integer, U<:Real}


  Fields
  ≡≡≡≡≡≡≡≡

  src    :: T<:Integer
  dst    :: T<:Integer
  weight :: U<:Real


  Supertype Hierarchy
  ≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡≡

  SimpleWeightedEdge{T<:Integer, U<:Real} <: AbstractSimpleWeightedEdge{T<:Integer} <: AbstractEdge{T<:Integer} <: Any


julia> my_mst[1].weight
3.1273492392134963
3

Spectacular. I’m not sure I would have understand the syntax even looking at the REPL. But this definitely will help for future issues. Thanks!

4

Glad that it solved your problem, don’t hesitate to mark the answer as a solution so that others know they don’t need to pile on :wink:
I deduced the solution by looking at the fields of the SimpleWeightedEdge type, which can be accessed with the object.field syntax. The help mode of the REPL, which I used to do that, can be accessed by typing ? as the first character

5

It’s generally considered bad practice to access internal fields directly, you should prefer the accessor weight(e::AbstractSimpleWeightedEdge). The documentation can probably be improved in this regard.

1 Like
6

Actually @JedPhillips maybe it would be better to mark the answer by @etienne_dg as the right solution